FIELD OF THE INVENTION
The invention relates to sidelobe canceller systems, MTI systems and other systems that decorrelate signals by removing correlated components. More specifically, the invention relates to a preprocessing arrangement of cancellers for cancellation of correlated signals.
Generally, a signal-processing system is designed to reduce the presence of undesired signals received together with desired signals. An example of this type of system is a sidelobe canceller system. In a conventional sidelobe canceller system, the main-radar antenna receives desirable target reflections through a high-gain main lobe and undesirable interference or jamming signals from M sources through low-gain sidelobes. These signals are all mixed in the antenna output.
Omnidirectional auxiliary antennas receive interference components which are correlated with the interference components of the main radar antenna signal. A sidelobe canceller system uses the auxiliary antenna signals to cancel the correlated components from the main radar antenna signal. At least M auxiliary antennas must be used to cancel M interference components.
Similarily, in MTI systems the received radar-return pulses include desired target reflections and undesired clutter from the sea surface, slow-moving rain squalls, chaff, etc. The radar clutter is correlated from pulse to pulse while the target reflections are moving quickly and have a phase change from pulse to to pulse, thus being uncorrelated. The MTI system cancels all correlated signals on a pulse-to-pulse basis, thus cancelling clutter, and passes uncorrelated signals, thus passing target reflections. The MTI system must use M +1 pulses to cancel M sources of clutter.
In the past, it was known how to build a canceller system having two auxiliary channels which would optimally cancel all correlated signal components between a main signal and two auxiliary signals. However, it was not known how to build a system having three or more auxiliary channels which would cancel all correlated components between the main channel and all auxiliary channels. For example, in a three auxiliary channel system, it was known how to decorrelate all components between the main channel and the first and second channels. However, in attempting to decorrelate the main channel from the third auxiliary channel it was not known how to remove both the first and second channel correlated components from the third channel. When removing either the first or second channel correlated components from the third channel the correlated components from the other of the two channels was reintroduced. Several configurations of canceller schemes were tried including those in U.S. Pat. No. 3,938,153 and 3,938,154.
What is needed is a canceller system which will provide optimum cancellation with the minimum number of cancellers when three or more auxiliary channels are used.